In general, the maraging steels have been used in many fields such as dies, rocket motor cases, load cells and gears, etc., which require high strength with high toughness. The conventional alloy system is 18% Ni--Co--Mo maraging steel of U.K. Pat. 936557. But, the Co and Mo, which are expensive alloying elements, increase the fabrication cost of maraging steels. Especially, the price of the Co, which is a strategic material, has increased rapidly due to the deficiency of supply in world market. Therefore, the demand for maraging steel of new composition has been increased in order to substitute the present alloys.
The Co-free 20-25% Ni maraging steel of U.K. Pat. 948354 was developed, but this steel has difficulty in commercialization due to poor toughness. The 20% Ni and 25% Ni maraging steels of France Pat. 2127799 have improved tensile strength to about 130 kg/mm.sup.2 and elongation to about 9%, however, they have poor ductility and the corrosion resistance was not considered. The Co-free maraging steel of Korean Pat. Publication No. 87-2074 is known, but this steel include Mo, which is an expensive alloying elements, and have poor ductility without considering corrosion resistance. On the other hand, the 18% Ni maraging steel has higher resistance to stress corrosion cracking and hydrogen embrittlement compared with the medium carbon low alloy steels, while, if it is used in corrosive atmosphere or used as part contacting with water, then the protective surface treatment is necessary. Therefore, the stainless maraging steel(IN-736), which have improved toughness and corrosion resistance instead of lowered strength compared with conventional maraging steels, was developed, however, this include Mo and the corrosion resistance is not good. While, the Co-and MO-free maraging steel having high strength and high toughness was developed as Korean Pat. Publication No. 90-402 by one of our inventors, but this steel has poor ductility and corrosion resistance.